It's difficult to determine the cause of death in this case, though we suspect that, at least in part, planting too deeply and perhaps accidental over- or under-watering may have contributed. Redbuds are sensitive to soil that's too wet or poorly-drained, but many young trees are installed with their root flare too deep (as it was in the nursery container as well). The linked page discusses planting depth, but essentially the root flare (where main roots branch off of the trunk base) should sit just at the soil surface; often, potted or burlapped trees wind-up having their root flare buried several inches below the soil surface. This predisposes them to root stress and dieback, sometimes several years after planting.

You can examine the trunk base below the leaf mulch for indications of vole gnawing. Their feeding damage on the bark of shrubs and trees can cause serious dieback. While the presence of a tiny amount of new growth low on the trunk suggests this is not a factor here, it's still worth investigating.

Redbuds are vulnerable to a few maladies. Verticillium Wilt and Botryospheria canker are two common infections, though these fungi tend to take advantage of easier-to-overwhelm weakened host trees stressed by taxing growing conditions. Ambrosia Beetle can be a serious but secondary pest (also often attacking trees already under stress, even if symptoms are not obvious). Deer damage from antler rubbing on the bark is another common cause of demise of young trees in general. We don't see any insects in the photos, but feel free to send more images with close-ups if you find what you suspect are pests.

There is no real recourse right now except to replace the tree or, if you'd rather see what the lone sprout matures into (more of a shrub than a tree, most likely), to give it time to regrow. Dead wood needs to be trimmed away in that case, since it cannot reanimate. This assumes the roots are still healthy enough to support that regrowth, though, which we doubt given how much growth has been lost so far.

If you replant, monitor the tree for watering needs by feeling the soil about six inches deep and watering only when it's becoming somewhat dry to the touch at that depth. New shrubs and trees should be monitored for at least their full first growing season in the ground, if not two seasons, to establish well. With a good head start like this, they become more self-sufficient as they mature and better able to handle the occasional stress from drought or wet spells, as long as a nearby roof downspout outlet or other water source isn't inundating the root zone with extra water.

Miri

Thank you so much!

You're welcome!